A lithium secondary battery is an energy storage device for storing electrical energy therein while lithium moves from an anode to a cathode during discharge and lithium ions move from a cathode to an anode during charge. When compared with other batteries, the lithium secondary battery has high energy density and small degree of self-discharge, and is widely used in various areas in general.
The constituent of the lithium secondary battery includes a cathode, an anode, an electrolyte and a separator. In early days, a lithium metal has been used as an anode active material in the lithium secondary battery, however has been replaced with a carbon-based material such as graphite because of the appearance of safety problems according to the repetition of charge and discharge. Since a carbon-based anode active material has similar electrochemical reaction potential with lithium ions as the lithium metal, and shows small change of crystalline structure during the continuous intercalation and deintercalation of the lithium ions, continuous charge and discharge may be possible and good charge and discharge lifetime may be realized.
However, in line with the expansion of the market from a small-sized lithium secondary battery used in a mobile phone to a large-sized secondary battery used in an automobile, techniques of increasing the capacity and the power of an anode active material are necessary. Thus, researches on developing a non-carbon-based anode active material are actively conducted mainly with silicon, tin, germanium, zinc, lead, or the like, which has higher theoretical capacity than the carbon-based anode active material.
These anode active materials may improve charge and discharge capacity and may increase energy density, however, since dendrite or a nonconductive material may be produced on an electrode by the repetition of the charge and discharge, the charge and discharge may be deteriorated, or expansion and contraction during the intercalation and deintercalation of lithium ions may be in a large scale. In this case, the secondary battery using the anode active material may insufficiently maintain discharge capacity according to the repetition of charge and discharge (hereinafter referred to as cycle properties), and may have an insufficient ratio of initial discharge capacity and initial charge capacity (discharge capacity/charge capacity, hereinafter referred to as initial efficiency).
Based on the above-described background, the present inventors have studied on a silicon-based anode active material capable of improving battery capacity as well as battery stability and cycle properties (lifetime characteristics), completed the present invention after securing that silicon oxide (second silicon oxide) would be prepared by heat treating silicon oxide (first silicon oxide) containing a relatively large amount of oxygen in a reducing atmosphere, and that a secondary battery using the silicon oxide as an anode active material had good battery stability and cycle properties (lifetime characteristics) while improving battery capacity.